Generally, a semiconductor light emitting device generates light when a forward current is applied thereto such as light emitting diode (LED) and a laser diode (LD). Especially, such an LED and LD have a p-n junction and a current applied to those light emitting devices is converted into a photon, to emit light from the devices.
The lights emitted from the LED and the LD may include various lights at a long wavelength to a short wavelength based on the types of the semiconductor devices. Above all, an LED fabricated of a semiconductor having a wide band gap can be used for realizing red, green and blue colors that consists of a visible area. Accordingly, those LEDs have been broadly and industrially applied to displaying parts of various electronic devices, streetlights and various display light source devices. In recent, a white light source has been developed through the LED and it is certain that the LEDs will be broadly used in light source devices for next generation common lighting.
Generally, a III-V nitride based semiconductor is fabricated after heteroepitaxially growing on an initial growth substrate to gain a good quality semiconductor thin film. Sapphire, SiC and Si have been used for such an initial growth substrate.
Out of the materials used for the initial growth substrate, a sapphire substrate has a predetermined lattice constant and thermal expansion coefficient that are quite different from a lattice constant and thermal expansion coefficient of the III-V nitride based semiconductor. Accordingly, it is difficult to layer multi-layered light emitting structure thin films configured of a good quality III-V nitride based semiconductor. In addition, the sapphire substrate has a deteriorating thermal conductivity and it results in having a disadvantage of failure in applying a high current to an LED. Also, the sapphire substrate is an electric insulator that has difficulties in dealing with external static electricity and it has a considerable disadvantage of potential error possibility that might be caused by the static electricity. Those disadvantages might deteriorate device reliability and cause quite limitation on a packaging process.
Moreover, the sapphire substrate that is an electric insulator has a MESA-structure formed in the same direction as a layering (growing) direction of a multilayered light emitting structure thin film configured of an n-type ohmic contact electrode and a p-type ohmic contact electrode. Also, the sapphire substrate requires an LED chip area that is a predetermined value or more. Accordingly, there is limitation on reducing the LED chip area and restriction on enhancing the LED chip production output per 2-inch wafer.
Rather than those disadvantages, the MESA-structured LED fabricated on the sapphire substrate that is the initial growth substrate has a further disadvantage of difficulties in dissipating much heat necessarily generated during the driving of a light emitting device, because of bad thermal conductivity.
As a result, it is limited to apply the MESA structure having sapphire attached thereto to the light emitting device used for large-area and high-capacity (namely, high-currents) such as a large-sized display and a light source for general lighting. In other words, if high currents are injected to a light emitting device for a long time, much heat will be generated and the heat will make the temperature inside a light emitting active layer to gradually increase. Accordingly, there might be a disadvantage of gradually decreased LED light emitting efficiency.
Different from the sapphire substrate, a silicon carbide (SiC) substrate has good thermal and electric conductivity. The lattice constant and thermal expansion coefficient of the III-V nitride based semiconductor are similar to a lattice constant and thermal expansion coefficient of the SiC that are important factors when growing a good quality semiconductor single-crystal thin film. Accordingly, a good quality light emitting structure thin film may grow successfully and various types of vertical-structure light emitting devices have been fabricated, using the multilayer light emitting structure thin film.
However, it is not easy to fabricate the good quality SiC growth substrate and the SiC growth substrate is quite high-cost, compared with the other growth substrates such as the sapphire substrate, such that it may have much limitation on application to mass production.
As a result, considering current technological, economic and efficient aspects, it is preferable to fabricate a light emitting device by using the multilayer light emitting structure thin film including the III-V nitride based semiconductor growing and disposed on the sapphire substrate.
To solve the disadvantages of the MESA-structure LED fabricated by using the multilayer light emitting structure thin film configured of the III-V nitride based semiconductor growing and disposed on the initial growth substrate as mentioned above, recently, there have been a lot of effort put into fabricating a vertical-structure light emitting structure by using the multilayer light emitting structure thin film consisting of the III-V nitride based semiconductor that is separated from the initial growth substrate safely after the multilayer light emitting structure thin film consisting of a good quality III-V nitride based semiconductor grows on an initial growth substrate such as a sapphire substrate, SiC substrate and Si substrate.
FIG. 1 is a sectional view illustrating a process of separating a sapphire substrate based on Laser Lift-Off technique according to a related art.
As shown in FIG. 1, a laser beam as a strong energy source is irradiated to a back side of a transparent sapphire substrate 110 based on Laser Lift-Off technique. After that, laser beam absorption is strongly generated in an interface 120 and 900° C. or more may be temporarily generated to perform thermochemical decomposition. Accordingly, a nitride thin film 130 is separated from the sapphire substrate 110.
However, as mentioned in many related documents, the multilayer light emitting structure consisting of the III-V nitride based semiconductor cannot endure a mechanical stress generated between the III-V nitride based semiconductor thin film and the thick sapphire substrate because of the different lattice constants and thermal expansion coefficients, in the Laser Lift-Off process. Accordingly, more damage and braking might be generated on a semiconductor single-layer thin film separated from the sapphire.
If the damage and braking is generated in the multilayered light emitting structure thin film, many leaky currents might be generated and also, a chip yield of light emitting devices such as LEDs might be degraded and entire performance of LED chips as light emitting devices might be degraded. Accordingly, there have been steadily studied on a sapphire substrate separation process that is able to minimize damage on the multilayered light emitting structure thin film and on a fabricating process of a vertical-structure LED, using the separated multilayered light emitting structure thin film.
As a result, when separating the sapphire substrate in the laser lift-off process, there have been suggested various proposals to minimize the damage and braking of the multilayered light emitting structure consisting of the III-V nitride based semiconductor thin films.
FIG. 2 is a sectional view illustrating a process of forming a conductive support strongly boned in a growth direction by performing a wafer bonding process and an electroplating process before the laser lift-off process, according to the related art to prevent the damage and braking of the multilayered light emitting structure thin film.
In reference to FIG. 2(a), before separating the multilayered light emitting structure thin film 220 and 230 consisting of the III-V nitride based semiconductor thin films after irradiating a laser beam via a back side of a transparent sapphire substrate 210, a wafer bonding process is performed on a bonding layer 240a form a strongly bonded conductive support 250 in a safe structure. Also, in reference to FIG. 2(b), before separating the multilayered light emitting structure thin films 220 and 230 from the sapphire substrate 210, an electroplating process is performed on a seeding layer 240b to form a strongly bonded conductive support 250 in a safe structure.
FIG. 3 is a sectional view illustrating a vertical-structure the III-V nitride based semiconductor light emitting device fabricated by using the conductive support strongly bonded in a safe structure with respect to the LLO process, according to the related art using the method of FIG. 2.
FIG. 3(a) is a sectional view illustrating a semiconductor light emitting device fabricated by using the method of forming the conductive support shown in FIG. 2(a). in reference to FIG. 3(a) illustrating the sectional view of the vertical light emitting device using the wafer bonding, the vertical light emitting device sequentially includes a conductive support 310 as a thermal and electrical insulator, a multilayered metal layer 330 having a bonding layer 320a and a p-type ohmic contact electrode 350, a p-type semiconductor clad layer 340, a light emitting active layer 350, n-type semiconductor clad layer 360, and a n-type ohmic contact layer 370. A semiconductor wafer having good thermal and electrical conductivity is used for the support 310 such as Si, Ge, SiGe and GaAs.
However, there is a big difference between the thermal expansion coefficient of the support 310 used in the vertical-structure light emitting device shown in FIG. 3(a) and that of the sapphire substrate having the multilayered light emitting structure thin films 340˜360 growing thereon. Accordingly, when Si or other conductive support is bonded according to wafer bonding, there might be wafer warpage and many micro-cracks generated in the multilayered light emitting structure thin film. That conventional light emitting structure has a disadvantage of difficulties in performing the processes and another disadvantage of a low production yield generated by the low performance thereof.
Meanwhile, FIG. 3(b) is a sectional view illustrating a semiconductor light emitting device fabricated according to the method of forming the conductive support shown in FIG. 2(b). In reference to FIG. 3(b) showing the sectional view of the vertical-structure light emitting device using the electroplating, the vertical-structure light emitting device fabricated by using the laser lift-off process and the electroplating process has the same structure, except a seeding layer 320b instead of the bonding layer 320a shown in FIG. 3(a). In this instance, the support 310 is a metallic thick film formed by the electroplating. A single metal such as Cu, Ni, W, Au and MO or alloy of those metals having good thermal electric conductivity may be used for the metallic thick film.
The support 310 provided in the light emitting device having the structure mentioned above shown in FIG. 3(b) is the metallic or alloyed thick film fabricated by the electroplating. Accordingly, the support 310 is more related to the big thermal expansion coefficient than the sapphire growth substrate, only to generate many problems of burr, wafer warpage and braking in the chip fabrication processes including mechanical sawing and laser scribing.
As a result, there are demands for a heat sink used in a vertical-structure semiconductor light emitting device that can solve the disadvantages of wafer warpage and braking, micro-crack generation, limitation on post-processes including the annealing and the chip fabrication and a low production yield, when fabricating the vertical-structure III-V nitride based semiconductor light emitting device by using the sapphire substrate separation process.